This invention relates generally to protective coatings for plastic articles and to methods for making hard, flexible thermoplastic articles.
Engineering resins are well-known, commercially available materials possessing physical and chemical properties which are useful in a wide variety of applications. For example, polycarbonate has replaced glass in many products because of its excellent breakage resistance. Exemplary products made from polycarbonates include automobile head lamps and stoplight lenses; safety shields in windows, architectural glazing, and the like. However, a major defect exhibited by polycarbonates is their very low scratch-resistance.
Scratch-resistant coatings for plastics have been available in the prior art. As an example, H. Clark discloses an unpigmented coating composition for transparent plastics in U.S. Pat. No. 4,027,073. These coatings contain a dispersion of colloidal silica in a lower aliphatic alcohol-water solution of the partial condensate of a silanol. The composition is maintained at a pH in the range of 3.0 to 6.0. While this coating formulation is useful in some applications, there still remains room for improvement, especially when the coated plastics are expected to take the place of metal and glass in many present day products.
Protective coatings having a basic pH and containing a colloidal dispersion of silica are also known in the art. These compositions usually contain the base sodium hydroxide to stabilize the dispersion and thereby prevent its agglomeration or gelation. However, sodium hydroxide is nonvolatile and will not vaporize from the coating composition upon cure. It is believed that the presence of sodium hydroxide and salts produced therefrom catalyzes hydrolysis reactions which result in the formation of cracks in the cured coating under conditions of heat, humidity, and ultraviolet light exposure.
In U.S. Pat. No. 4,624,870, B. Anthony eliminated the problem related to sodium hydroxide by discovering a silica-containing coating composition having an alkaline pH and stabilized by the use of a base which is volatile at curing temperatures. These coatings exhibit good physical properties under a variety of conditions.
As could be expected, the continuum of improvement in the performance of colloidal silica-based silicone coatings has encouraged those in the art to use the coatings in even more vigorous applications. For example, the coatings can be applied to large polycarbonate sheets which are then shaped and used as glazing, e.g., bus or subway car windows. These types of applications often require a high level of flexibility and weatherability while retaining excellent abrasion resistance.
As described in U.S. Pat. No. 4,159,206, an attempt was made by Armbruster et al. to impart weatherability and flexibility to colloidal silica-based coatings by the use of a mixture of dialkyldialkoxysilanes and alkyltrialkoxysilanes. While such coating compositions may provide an adequate combination of abrasion resistance and flexibility for some types of plastic articles, their overall performance falls short in some of today's more demanding product applications. For example, thermoplastic articles having such coatings applied thereon are sometimes shaped by thermoforming techniques, i.e., heating the sheet to its softening temperature and then forcing the hot material against the contours of a mold by mechanical or pneumatic techniques. While the substrate may be quite amenable to thermoforming, the coatings themselves are very susceptible to thermal and bending stresses, and frequently exhibit cracking after the shaped product is cooled to room temperature.
Furthermore, articles coated with these compositions continue to be subjected to harsh conditions throughout their life. For example, the articles are often subjected to heating/cooling cycles in an outdoor environment, resulting in stresses caused by the difference in the thermal expansion coefficient for the plastic and for the silicone coating. Moreover, the coatings may be susceptible to general degradation from the effects of weather and ultraviolet light exposure.
It is therefore an object of the present invention to provide a colloidal silica-based coating composition having a high level of abrasion resistance and improved resistance to cracking under exposure to thermal and mechanical stresses.
It is another object of the present invention to provide a protectively coated, abrasion-resistant plastic article which may be thermoformed without cracking or other degradation of the protective coating.
It is yet another object of the present invention to provide a method for applying smooth, hard, flexible coatings over a thermoplastic substrate.
It is still another object of the present invention to provide an abrasion-resistant, thermoformable plastic article having a protective top layer characterized by high abrasion resistance, flexibility, and freedom from microcracks.